Standard photodiodes are packaged along with a pre-amplifier in a transistor-outline (TO) can package. A TO can package is a hermetically sealed unit with a lens in one end for directing incoming light onto the photodiode, which converts the light into an electrical current. The conventional TO can package implementation of an optical receiver includes four leads, two of which provide transmission of power (Power and Ground) and two of which provide transmission of differential data (Out+ and Out−). Newly developed electro-optic devices, e.g. transceivers, offer diagnostic capabilities, such as optical power monitoring, which includes measuring the power level of the input optical signal. Monitoring the power level of incoming light provides an indication of the quality of the transmitting laser, as well as the incoming transmission line. Conventional optical power monitors are relatively complicated and costly. Moreover, the aforementioned basic requirements consume all of the four leads from the conventional TO can, leaving none for transmitting the optical power level or any other operating parameter. One option is to add additional leads; however, this option increases costs and makes assembly more difficult, particularly for small form factor (SFF) devices. Another option is to utilize the AC component of the photodiode current, such as disclosed in U.S. Pat. No. 6,528,777 issued Mar. 4, 2003 to Ames et al. Unfortunately, this system does not incorporate any feedback in the control system and can therefore develop large errors due to drift over the environmental corners.
An object of the present invention is to overcome the shortcomings of the prior art by providing a relatively simple and inexpensive optical power monitor for a photodiode in a conventional four-lead TO can package.
Another object of the present invention is to provide a means by which the photodiode bias current level is monitored using a common mode signal that can be transmitted using the existing four leads provided by a conventional TO can package.